Display

ABSTRACT

An active matrix display comprising a light control device and a field effect transistor for driving the light control device. The active layer of the field effect transistor comprises an amorphous.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display using an amorphous oxide foran active layer of a transistor.

2. Related Background Art

In recent years, flat panel displays (FPD's) have been put intopractical use owing to the progress of a technique for a liquid crystal,electroluminescence (EL), or the like.

Those FPD's are each driven by an active matrix circuit of a fieldeffect thin film transistor (TFT) using an amorphous silicon thin filmor a polycrystalline silicon thin film arranged on a glass substrate foran active layer.

Meanwhile, attempts have been made to use a resin substrate having alight weight and flexibility instead of a glass substrate for reducingthe thickness and weight of each of those FPD's and for improving theresistance to breakage thereof.

However, the production of a transistor using the above-describedsilicon thin film requires a step of heating at a relatively hightemperature, so it is generally difficult to directly form the siliconthin film on a resin substrate having low heat resistance.

In view of the foregoing, the development of a TFT using an oxidesemiconductor thin film made of, for example, ZnO that can be formedinto a film at a low temperature has been vigorously conducted (JP2003-298062 A).

Meanwhile, no technical level to develop an applied technology of aconventional TFT using an oxide semiconductor thin film has beenachieved probably because the TFT has no sufficient propertiescomparable to those of a TFT using silicon.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel display using atransistor using an oxide for its active layer.

According to a first gist of the present invention, there is provided anactive matrix display including:

a light control device; and

a field effect transistor for driving the light control device,

in which an active layer of the field effect transistor is an amorphousoxide having an electron carrier concentration of less than 10¹⁸/cm³.

According to a second gist of the present invention, there is providedan active matrix display including:

a light control device; and

a field effect transistor for driving the light control device,

in which an active layer of the field effect transistor is an amorphousoxide of which the electron mobility tends to increase with increasingelectron carrier concentration.

According to a third gist of the present invention, there is provided anactive matrix display including:

a light control device; and

a field effect transistor for driving the light control device,

in which an active layer of the field effect transistor has an amorphousoxide semiconductor capable of realizing normally-off of the fieldeffect transistor.

The inventors of the present invention have made investigation into anoxide semiconductor. As a result, they have found that ZnO cannotgenerally form a stable amorphous phase. In addition, most ZnO shows apolycrystalline phase. Therefore, a carrier is scattered at an interfacebetween polycrystalline particles, with the result that electronmobility cannot be increased.

In addition, an oxygen defect is apt to enter ZnO. As a result, a largenumber of carrier electrons are generated, so it is difficult to reduceelectric conductivity. It has been found that, owing to the foregoing,even when no gate voltage is applied to a transistor, a large currentflows between a source terminal and a drain terminal, so a normally-offoperation of a TFT cannot be realized. It seems also difficult toincrease on-off ratio of the transistor.

In addition, the inventors of the present invention have examined anamorphous oxide film Zn_(x)M_(y)In_(z)O_((x+3y/2+3z/2)) (where Mrepresents at least one element of Al and Ga) described in JP2000-044236 A. The material has an electron carrier concentration of10¹⁸/cm³ or more, so it is suitable for a mere transparent electrode.

However, it has been found that, when an oxide having an electroncarrier concentration of 10¹⁸/cm³ or more is used for a channel layer ofa TFT, sufficient on-off ratio cannot be secured, so the oxide is notappropriate for a normally-off TFT.

That is, a conventional amorphous oxide film has been unable to providea film having an electron carrier concentration of less than 10¹⁸/cm³.

In view of the foregoing, the inventors of the present invention haveproduced a TFT using an amorphous oxide having an electron carrierconcentration of less than 10¹⁸/cm³ for an active layer of a fieldeffect transistor. As a result, they have obtained a TFT having desiredproperties, and have discovered that the TFT is applicable to a display.

The inventors of the present invention have conducted vigorous researchand development concerning InGaO₃(ZnO)_(m) and conditions under whichthe material is formed into a film. As a result, they have found that anelectron carrier concentration of less than 10¹⁸/cm³ can be achieved bycontrolling the conditions of an oxygen atmosphere upon film formation.

The present invention relates to a display using a film that hasrealized a desired electron carrier concentration.

Hereinafter, the present invention will be described in detail.

The light control device to be used in the present invention is anon-selfluminous electro-optic element such as a liquid crystal deviceor a device containing an electrophoretic particle.

The light control device may be constituted by means of a liquidcrystal, and an orientation film and an insulation film may be arrangedin this order from the side of the liquid crystal between the activelayer and the liquid crystal.

The light control device may be constituted by means of a liquidcrystal, and an orientation film and an insulation film may be arrangedin the stated order from the side of the liquid crystal between the gateelectrode of the field effect transistor and the liquid crystal.

The insulation film is, for example, a silicon oxide film or a siliconnitride film.

The present invention relates to a light control device with its lighttransmittance or light reflectance controlled by an output from a fieldeffect transistor.

In addition, an output terminal of a field effect transistor having, asan active layer, an amorphous semiconductor containing In, Ga, Zn, and Oand having an electron carrier concentration of less than 10¹⁸/cm³ isconnected to an electrode of a light transmittance control device or ofa light reflectance control device.

In further aspect of the present invention, the light-emitting device isan electroluminescent device.

In further aspect of the present invention, the light transmittancecontrol device or the light reflectance control device is a liquidcrystal cell.

In further aspect of the present invention, the light transmittancecontrol device or the light reflectance control device is anelectrophoretic particle cell.

In further aspect of the present invention, the electrophoretic particlecell is a cell having counter electrodes and a capsule in which a fluidand a particle are sealed, the capsule being sandwiched between thecounter electrodes.

In further aspect of the present invention, the light control device isarranged on a flexible resin substrate.

In further aspect of the present invention, the light control device isarranged on a light transmissive substrate.

In further aspect of the present invention, the multiple light controldevices are arranged two-dimensionally together with the multiple fieldeffect transistors wired in an active matrix manner.

According to another aspect of the present invention, there is provideda broadcasting dynamic image display device such as a televisionreceiving set including the above-described display.

According to another aspect of the present invention, there is provideda digital information processing device such as a computer including theabove-described display.

According to another aspect of the present invention, there is providedportable information equipment such as a cellular phone, a portablemusic reproducer, or a portable dynamic image reproducer including theabove-described display.

According to another aspect of the present invention, there is providedan image pickup device such as a still camera or a movie cameraincluding the above-described display for a viewfinder of the imagepickup device, for observing a photographed image, for displayingphotography information, or for other purposes.

According to another aspect of the present invention, there is provideda building structure such as a window, a door, a ceiling, a floor, aninner wall, an outer wall, or a partition including the above-describeddisplay for displaying an image on the surface of the buildingstructure.

According to another aspect of the present invention, there is provideda structure such as a window, a door, a ceiling, a floor, an inner wall,an outer wall, or a partition for a movable body such as a vehicle, anairplane, or a ship including the above-described display for displayingan image on the surface of the structure.

According to another aspect of the present invention, there is providedan advertising device such as advertising means in a vehicle of a publictransportation, or a signboard or advertising tower in a city includingthe above-described display for displaying an image on the advertingdevice.

According to another aspect of the present invention, there is provideda light control device including: a light transmittance control device;a light reflectance control device; and a field effect transistorhaving, as an active layer, an amorphous oxide having an electroncarrier concentration of less than 10¹⁸/cm³ at room temperature, inwhich an output terminal of the field effect transistor is connected toan electrode of the light transmittance control device or of the lightreflectance control device.

According to another aspect of the present invention, there is provideda light control device including: a light transmittance control device;a light reflectance control device; and a field effect transistorhaving, as an active layer, an amorphous oxide whose electron mobilityincreases with increasing electron carrier concentration, in which anoutput terminal of the field effect transistor is connected to anelectrode of the light transmittance control device or of the lightreflectance control device.

According to a forth gist of the present invention, there is provided anactive matrix display comprising:

a first electrode;

a second electrode;

a liquid crystal interposed between the first and second electrodes; and

a field effect transistor for driving the liquid crystal,

wherein an active layer of the transistor comprises an amorphous oxide,and

wherein the transistor is a normally-off transistor.

The amorphous oxide is preferably an oxide containing In, Zn and Sn, anoxide containing In and Zn, an oxide containing In and Sn, or an oxidecontaining In.

In addition it is preferable that the active matrix display furthercomprises an orientation film and an insulation film arranged in thisorder from a side of the liquid crystal between a gate electrode of thefield effect transistor and the liquid crystal.

According to the present invention, there can be provided a noveldisplay.

As in one aspect of the present invention, when the display is used fora light transmissive substrate, for example, when the display is used asa light transmissive member such as a glass-type information equipment,a building structure, or a window of a movable body, a so-calledsee-through display can be provided.

The reason for the foregoing is as follows. The field effect transistoraccording to the present invention including its active layer istransparent, or has transmissivity, with respect to visible light.Therefore, external light that has passed through the window, and adisplay image on the device or device of the present invention can beseen on the same optical axis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relationship between the electron carrierconcentration of an In—Ga—Zn—O-based amorphous oxide formed into a filmby means of a pulse laser deposition method and an oxygen partialpressure upon film formation;

FIG. 2 is a graph showing a relationship between the electron carrierconcentration and electron mobility of an In—Ga—Zn—O-based amorphousoxide film formed by means of a pulse laser deposition method;

FIG. 3 is a graph showing a relationship between the electricconductivity of an In—Ga—Zn—O-based amorphous oxide film formed by meansof a high-frequency sputtering method using an argon gas and an oxygenpartial pressure upon film formation;

FIG. 4 shows graphs showing changes in electric conductivity, electroncarrier concentration, and electron mobility with the value for x ofInGaO₃(Zn_(1-x)Mg_(x)O)₄ formed into a film by means of a pulse laserdeposition method in an atmosphere having an oxygen partial pressure of0.8 Pa;

FIG. 5 is a schematic view showing a top gate TFT device structure;

FIG. 6 is a graph showing the current-voltage characteristics of a topgate TFT device using Y₂O₃ for a gate insulation film;

FIG. 7 is a schematic sectional view of a light control device;

FIG. 8 is a schematic sectional view of a light control device;

FIG. 9 is a schematic view showing a pulse laser deposition apparatus;and

FIG. 10 is a schematic view showing a sputter film forming apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

The active matrix display according to the present invention will bedescribed with reference to FIG. 8.

In the figure, reference numeral 11 denotes a base substance orsubstrate; 12, an amorphous oxide; 13, a source electrode; 14, a drainelectrode; 18, an electrode (pixel electrode); 15, a gate insulationfilm; 16, a gate electrode; 21 and 22, high-resistance films; and 23, aportion containing a liquid crystal or an electrophoretic particle. Theportion 23 constitutes part of the light control device in the presentinvention. Reference numeral 20 denotes an electrode or a substratehaving an electrode. An example of the electrode includes a transparentelectrode such as ITO.

In the present invention, an output terminal (corresponding to the drainelectrode 14) of a field effect transistor for driving the light controldevice constituted by a liquid crystal or the like is connected to theelectrode 18 constituting the light control device.

The light control device as employed in the present invention may bereferred to also as a light modulation device.

In addition, an amorphous oxide having an electron carrier concentrationof less than 10¹⁸/cm³ is used for the active layer 12 of the fieldeffect transistor.

An amorphous oxide whose electron mobility tends to increase withincreasing electron carrier concentration can also be used in thepresent invention.

When a liquid crystal is used for the light control device, thehigh-resistance film 21 or 22 serves as an orientation film (such aspolyimide) for orienting the liquid crystal.

In FIG. 8, reference numeral 17 denotes an interlayer insulation film.The insulation film 17 and the high-resistance film 21 may beconstituted by the same material.

However, the high-resistance film (orientation film) 21 and theinsulation film 17 are desirably formed of different materials forimproving their insulating properties. For example, silicon oxide orsilicon nitride is used for the insulation film 17.

In particular, it is preferable to use the insulation film 17constituted by silicon nitride or the like to separate the amorphousoxide or the gate insulation film 15 from the liquid crystal. This isbecause the penetration of unexpected atoms or ion species from theliquid crystal or the like into the gate insulation film and theamorphous oxide constituting the field effect transistor can besuppressed. The gate insulation film 15 and the insulation film 17 maybe constituted by different materials.

Any other layer except the orientation film may be interposed betweenthe liquid crystal and the insulation film 17.

(Amorphous Oxide)

The electron carrier concentration of the amorphous oxide according tothe present invention is a value measured at room temperature. Roomtemperature is, for example, 25° C., and, specifically, is a temperatureappropriately selected from the range of about 0° C. to 40° C. It shouldbe noted that there is no need for the electron carrier concentration ofthe amorphous oxide according to the present invention to have a valueof less than 10¹⁸/cm³ in the entire range of 0° C. to 40° C. Forexample, an electron carrier concentration of less than 10¹⁸/cm³ hasonly to be realized at 25° C. In addition, reducing the electron carrierconcentration to 10¹⁷/cm³ or less, or more preferably 10¹⁶/cm³ or lessprovides a normally-off TFT in high yield.

The electron carrier concentration of less than 10¹⁸/cm³ is preferablyan electron carrier concentration of less than 1×10¹⁸/cm³, morepreferably an electron carrier concentration of less than 1.0×10¹⁸/cm³.

The electron carrier concentration can be measured through Hall effectmeasurement.

The term “amorphous oxide” as used herein refers to an oxide having ahalo pattern to be observed, and showing no specific diffraction ray, inan X-ray diffraction spectrum.

The lower limit for the electron carrier concentration in the amorphousoxide of the present invention is not particularly limited as long asthe amorphous oxide is applicable to a channel layer of a TFT. The lowerlimit is, for example, 10¹²/cm³.

Therefore, in the present invention, as in each of the examples to bedescribed later, the electron carrier concentration is set to fallwithin the range of, for example, preferably 10¹²/cm³ (inclusive) to10¹⁸/cm³ (exclusive), more preferably 10¹³/cm³ to 10¹⁷/cm³ (bothinclusive), or still more preferably 10¹⁵/cm³ to 10¹⁶/cm³ (bothinclusive) by controlling the material, composition ratio, productionconditions, and the like of the amorphous oxide.

In addition to an InZnGa oxide, the amorphous oxide can be appropriatelyselected from an In oxide, an In_(x)Zn_(1-x) oxide (0.2≦x≦1), anIn_(x)Sn_(1-x) oxide (0.8≦x≦1), and an In_(x)(Zn, Sn)_(1-x) oxide(0.15≦x≦1).

The In_(x)(Zn, Sn)_(1-x) oxide can be described as anIn_(x)(Zn_(y)Sn_(1-y))_(1-x) oxide, and y ranges from 1 to 0.

Part of In in an In oxide containing none of Zn and Sn can be replacedwith Ga. That is, the In oxide can be turned into an In_(x)Ga_(1-x)oxide (0≦x≦1).

Hereinafter, an amorphous oxide having an electron carrier concentrationof less than 10¹⁸/cm³ that the inventors of the present invention havesucceeded in producing will be described in detail.

The oxide contains In—Ga—Zn—O, its composition in a crystalline state isrepresented by InGaO₃(ZnO)_(m) (where m represents a natural number ofless than 6), and its electron carrier concentration is less than10¹⁸/cm³.

The oxide contains In—Ga—Zn—Mg—O, its composition in a crystalline stateis represented by InGaO₃(Zn_(1-x)Mg_(x)O)_(m) (where m represents anatural number of less than 6 and 0<x≦1), and its electron carrierconcentration is less than 10¹⁸/cm³.

A film constituted by each of those oxides is also preferably designedto have an electron mobility in excess of 1 cm²/(V·sec).

The use of the film for a channel layer enables transistorcharacteristics including a gate current at the time of turning atransistor off of less than 0.1 μA (that is, normally off) and an on-offratio in excess of 10³. In addition, the use realizes a flexible TFTwhich is transparent, or has transmissivity, with respect to visiblelight.

The electron mobility of the film increases with increasing number ofconduction electrons. A glass substrate, a plastic substrate made of aresin, a plastic film, or the like can be used as a substrate forforming a transparent film.

When the amorphous oxide film is used for a channel layer, one of Al₂O₃,Y₂O₃, and HfO₂, or a mixed crystal compound containing at least twokinds of these compounds can be used for a gate insulation film.

It is also preferable to form the amorphous oxide into a film in anatmosphere containing an oxygen gas without intentionally adding anyimpurity ion for increasing electrical resistance to the amorphousoxide.

The inventors of the present invention have found that thesemi-insulating oxide amorphous thin film has specific property withwhich the electron mobility of the film increases with increasing numberof conduction electrons. Furthermore, the inventors have found that aTFT produced by means of the film is provided with additionally improvedtransistor characteristics including an on-off ratio, a saturationcurrent in a pinch-off state, and a switching speed. That is, theinventors have found that a normally-off TFT can be realized by using anamorphous oxide.

The use of the amorphous oxide thin film for a channel layer of a filmtransistor provides an electron mobility in excess of 1 cm²/(V·sec),preferably in excess of 5 cm²/(V·sec).

When the electron carrier concentration is less than 10¹⁸/cm³, orpreferably less than 10¹⁶/cm³, a current between drain and sourceterminals at the time of off (when no gate voltage is applied) can beset to be less than 10 μA, or preferably less than 0.1 μA.

The use of the film provides a saturation current after pinch-off inexcess of 10 μA and an on-off ratio in excess of 10³ when the electronmobility exceeds 1 cm²/(V·sec), or preferably exceeds 5 cm²/(V·sec).

In a TFT, a high voltage is applied to a gate terminal in a pinch-offstate, and electrons are present in a channel at a high density.

Therefore, according to the present invention, a saturation currentvalue can be increased by an amount corresponding to an increase inelectron mobility. As a result, improvements of transistorcharacteristics including an increase in on-off ratio, an increase insaturation current, and an increase in switching speed can be expected.

In a typical compound, when the number of electrons increases, electronmobility reduces owing to a collision between electrons.

Examples of a structure that can be used for the TFT include: astaggered (top gate) structure in which a gate insulation film and agate terminal are formed in order on a semiconductor channel layer; andan inversely staggered (bottom gate) structure in which a gateinsulation film and a semiconductor channel layer are formed in order ona gate terminal.

(First Film Forming Method: PLD Method)

The amorphous state of an amorphous oxide thin film whose composition ina crystalline state is represented by InGaO₃(ZnO)_(m) (where mrepresents a natural number of less than 6) is stably maintained up to ahigh temperature equal to or higher than 800° C. when the value of m isless than 6. However, as the value of m increases, that is, as the ratioof ZnO to InGaO₃ increases to cause the composition to be close to theZnO composition, the thin film is apt to crystallize.

Therefore, a value of m of less than 6 is preferable for a channel layerof an amorphous TFT.

A vapor phase deposition method involving the use of a polycrystallinesintered material having an InGaO₃(ZnO)_(m) composition as a target is adesirable film forming method. Of such vapor phase deposition methods, asputtering method and a pulse laser deposition method are suitable.Furthermore, a sputtering method is most suitable from the viewpoint ofmass productivity.

However, when the amorphous film is produced under typical conditions,an oxygen defect mainly occurs, so it has been unable to provide anelectron carrier concentration of less than 10¹⁸/cm³, that is, anelectric conductivity of 10 S/cm or less. The use of such film makes itimpossible to constitute a normally-off transistor.

The inventors of the present invention have produced In—Ga—Zn—O by meansof a pulse laser deposition method with the aide of an apparatus shownin FIG. 9.

Film formation was performed by means of such PLD film forming apparatusas shown in FIG. 9.

In the figure, reference numeral 701 denotes a rotary pump (RP); 702, aturbo-molecular pump (TMP); 703, a preparatory chamber; 704, an electrongun for RHEED; 705, substrate holding means for rotating, and movingvertically, a substrate; 706, a laser entrance window; 707, thesubstrate; 708, a target; 709, a radical source; 710, a gas inlet; 711,target holding means for rotating, and moving vertically, the target;712, a bypass line; 713, a main line; 714, a turbo-molecular pump (TMP);715, a rotary pump (RP); 716, a titanium getter pump; and 717, ashutter. In addition, in the figure, reference numeral 718 denotes anion vacuum gauge (IG); 719, a Pirani vacuum gauge (PG); 720, a baratronvacuum gauge (BG); and 721, a growth chamber (chamber).

An In—Ga—Zn—O-based amorphous oxide semiconductor thin film wasdeposited on an SiO₂ glass substrate (1737 manufactured by Corning Inc.)by means of a pulse laser deposition method using a KrF excimer laser.Prior to the deposition, the substrate was subjected to degreasingwashing by means of an ultrasonic wave for 5 minutes in each of acetone,ethanol, and ultrapure water, and was then dried in the air at 100° C.

An InGaO₃(ZnO)₄ sintered material target (having a diameter of 20 mm anda thickness of 5 mm in size) was used as the polycrystalline target. Thetarget was produced by wet-mixing 4N reagents of In₂O₃, Ga₂O₃, and ZnOas starting materials in ethanol as a solvent; calcining the mixture at1,000° C. for 2 hours; dry-pulverizing the resultant; and sintering thepulverized product at 1,550° C. for 2 hours. The target thus producedhad an electric conductivity of 90 (S/cm).

Film formation was performed with the ultimate pressure in the growthchamber set to 2×10⁻⁶ (Pa) and an oxygen partial pressure during growthcontrolled to be 6.5 (Pa).

The oxygen partial pressure in the chamber 721 was 6.5 Pa and thesubstrate temperature was 25° C.

The distance between the target 708 and the deposition substrate 707 was30 (mm), and the power of the KrF excimer laser incident from theentrance window 706 was in the range of 1.5 to 3 (mJ/cm²/pulse). Thepulse width, pulse rate, and irradiation spot diameter were set to 20(nsec), 10 (Hz), and 1×1 (mm square), respectively.

Thus, film formation was performed at a film forming rate of 7 (nm/min).

X-ray diffraction was conducted on the resultant thin film by means of asmall angle X-ray scattering method (SAXS) (thin film method, angle ofincidence 0.5 degree). As a result, no clear diffraction peak wasobserved. Therefore, the produced In—Ga—Zn—O-based thin film can be saidto be amorphous.

Furthermore, X-ray reflectance measurement was performed, and patternanalysis was performed. As a result, the thin film was found to have amean square roughness (Rrms) of about 0.5 nm and a thickness of about120 nm. X-ray fluorescence (XRF) analysis confirmed that the metalcomposition ratio of the thin film was In:Ga:Zn=0.98:1.02:4.

The film had an electric conductivity of less than about 10⁻² S/cm. Theelectron carrier concentration and electron mobility of the film areestimated to be about 10¹⁶/cm³ or less and about 5 cm²/(V·sec),respectively.

Owing to the analysis of a light absorption spectrum, the forbidden bandenergy width of the produced amorphous thin film was determined to beabout 3 eV. The foregoing shows that the produced In—Ga—Zn—O-based thinfilm is a transparent and flat thin film showing an amorphous phaseclose to the composition of InGaO₃(ZnO)₄ as a crystal, having littleoxygen defect, and having a small electric conductivity.

Specific description will be made with reference to FIG. 1. The figureshows the change of the electron carrier concentration of a transparentamorphous oxide thin film formed with changing oxygen partial pressureunder the same conditions as those of this embodiment which is composedof In—Ga—Zn—O and has a composition in an assumed crystalline staterepresented by InGaO₃(ZnO)_(m) (where m represents a number of less than6).

Film formation was performed in an atmosphere having a high oxygenpartial pressure in excess of 4.5 Pa under the same conditions as thoseof this embodiment. As a result, as shown in FIG. 1, it was able toreduce the electron carrier concentration to less than 10¹⁸/cm³. In thiscase, the substrate had a temperature maintained at a temperature nearlyequal to room temperature unless intentionally heated. The substratetemperature is preferably kept at a temperature lower than 100° C. inorder to enable a flexible plastic film to be used as a substrate.

Additionally increasing the oxygen partial pressure can additionallyreduce the electron carrier concentration. For example, as shown in FIG.1, an InGaO₃(ZnO)₄ thin film formed at a substrate temperature of 25° C.and an oxygen partial pressure of 5 Pa had a number of electron carriersreduced to 10¹⁶/cm³.

As shown in FIG. 2, the resultant thin film had an electron mobility inexcess of 1 cm²/(V·sec). However, in the pulse laser deposition methodof this example, when the oxygen partial pressure is 6.5 Pa or more, thesurface of the deposited film becomes irregular, so it becomes difficultto use the film as a channel layer of a TFT.

Therefore, a normally-off transistor can be constituted by using atransparent amorphous oxide thin film having a composition in acrystalline state represented by InGaO₃(ZnO)_(m) (where m represents anumber of less than 6) by means of a pulse laser deposition method in anatmosphere having an oxygen partial pressure in excess of 4.5 Pa, ordesirably in excess of 5 Pa and less than 6.5 Pa.

In addition, the thin film had an electron mobility in excess of 1cm²/V·sec, so the on-off ratio was able to exceed 10³.

As described above, when an InGaZn oxide is formed into a film by meansof the PLD method under the conditions shown in this embodiment, anoxygen partial pressure is desirably controlled to be 4.5 Pa or more andless than 6.5 Pa.

The realization of an electron carrier concentration of less than10¹⁸/cm³ depends on, for example, a condition of an oxygen partialpressure, the structure of a film forming apparatus, and a material anda composition to be formed into a film.

Next, an amorphous oxide was produced at an oxygen partial pressure of6.5 Pa in the above apparatus, and then a top gate MISFET device shownin FIG. 5 was produced. To be specific, at first, a semi-insulatingamorphous InGaO₃(ZnO)₄ film having a thickness of 120 nm to be used as achannel layer (2) was formed on a glass substrate (1) by means of theabove-described method of producing an amorphous In—Ga—Zn—O thin film.

Then, InGaO₃(ZnO)₄ and a gold film each having a large electricconductivity and a thickness of 30 nm were laminated on the film bymeans of a pulse laser deposition method with the oxygen partialpressure in the chamber set to be less than 1 Pa, to thereby form adrain terminal (5) and a source terminal (6) by means of aphotolithography method and a lift-off method. Finally, a Y₂O₃ film tobe used as a gate insulation film (3) (thickness: 90 nm, relativedielectric constant: about 15, leak current density: 10⁻³ A/cm² uponapplication of 0.5 MV/cm) was formed by means of an electron beamdeposition method. A gold film was formed on the Y₂O₃ film, to therebyform a gate terminal (4) by means of a photolithography method and alift-off method.

Evaluation of MISFET Device for Characteristics

FIG. 6 shows the current-voltage characteristics of an MISFET devicemeasured at room temperature. The fact that a drain current I_(DS)increased with increasing drain voltage V_(DS) shows that the channel isan n-type semiconductor. This is not in contradiction to the fact thatan amorphous In—Ga—Zn—O-based semiconductor is of an n-type. I_(DS)saturated (pinched off) at V_(DS) of about 6 V. The saturation is atypical behavior of a semiconductor transistor. Investigation into again characteristic showed that the threshold value for a gate voltageV_(GS) was about −0.5 V upon application of V_(DS)=4 V. A currentI_(DS)=1.0×10⁻⁵ A flowed when V_(G)=10 V. This corresponds to the factthat a gate bias enabled a carrier to be induced in an In—Ga—Zn—O-basedamorphous semiconductor thin film as an insulator.

The transistor had an on-off ratio in excess of 10³. The field effectmobility was calculated from an output characteristic. As a result, afield effect mobility of about 7 cm²(Vs)⁻¹ was obtained in a saturationregion. The produced device was irradiated with visible light to performsimilar measurement. However, no changes in transistor characteristicswere observed.

According to this embodiment, a thin film transistor having a channellayer with a small electron carrier concentration (that is, a highelectrical resistance) and large electron mobility can be realized.

The above-described amorphous oxide had excellent properties. That is,electron mobility increased with increasing electron carrierconcentration, and degenerate conduction was exhibited.

In this embodiment, a thin film transistor was formed on a glasssubstrate. A substrate such as a plastic plate or a film can also beused because film formation itself can be performed at room temperature.

In addition, the amorphous oxide obtained in this embodiment absorbsnearly no visible light and can realize a transparent and flexible TFT.

(Second Film Forming Method: Sputtering Method (SP Method))

Description will be given of film formation by means of a high-frequencySP method using an argon gas as an atmospheric gas.

The SP method was performed by means of an apparatus shown in FIG. 10.In the figure, reference numeral 807 denotes a deposition substrate;808, a target; 805, substrate holding means equipped with a coolingmechanism; 814, a turbo-molecular pump; 815, a rotary pump; 817, ashutter; 818, an ion vacuum gauge; 819, a Pirani vacuum gauge; 821, agrowth chamber (chamber); and 830, a gate valve.

An SiO₂ glass substrate (1737 manufactured by Corning Inc.) was preparedas the deposition substrate 807. Prior to film formation, the substratewas subjected to degreasing washing by means of an ultrasonic wave for 5minutes in each of acetone, ethanol, and ultrapure water, and was thendried in the air at 100° C.

A polycrystalline sintered material having an InGaO₃(ZnO)₄ composition(having a diameter of 20 mm and a thickness of 5 mm in size) was usedfor a target material.

The sintered material was produced by wet-mixing 4N reagents of In₂O₃,Ga₂O₃, and ZnO as starting materials in ethanol as a solvent; calciningthe mixture at 1,000° C. for 2 hours; dry-pulverizing the resultant; andsintering the pulverized product at 1,550° C. for 2 hours. The target808 had an electric conductivity of 90 (S/cm), and was in asemi-insulating state.

The ultimate pressure in the growth chamber 821 was 1×10⁻⁴ (Pa) and thetotal pressure of an oxygen gas and the argon gas during growth wasmaintained at a constant value in the range of 4 to 0.1×10⁻¹ (Pa) Then,the ratio between the partial pressure of the argon gas and the oxygenpartial pressure was varied to change the oxygen partial pressure in therange of 10⁻³ to 2×10⁻¹ (Pa).

In addition, the substrate temperature was set to be room temperature,and the distance between the target 808 and the deposition substrate 807was 30 (mm).

Supplied power was RF180 W, and film formation was performed at a filmforming rate of 10 (nm/min).

X-ray diffraction was conducted on the resultant film by means of asmall angle X-ray scattering method (SAXS) (thin film method, angle ofincidence 0.5 degree). As a result, no clear diffraction peak wasdetected. Therefore, the produced In—Zn—Ga—O-based film was found to bean amorphous film.

Furthermore, X-ray reflectance measurement was performed, and patternanalysis was performed. As a result, the thin film was found to have amean square roughness (Rrms) of about 0.5 nm and a thickness of about120 nm. X-ray fluorescence (XRF) analysis confirmed that the metalcomposition ratio of the thin film was In:Ga:Zn=0.98:1.02:4.

The electric conductivity of the amorphous oxide film obtained wasmeasured with the oxygen partial pressure of the atmosphere during filmformation changed. FIG. 3 shows the results.

As shown in FIG. 3, film formation in an atmosphere having a high oxygenpartial pressure in excess of 3×10⁻² Pa was able to reduce an electricconductivity to less than 10 S/cm.

Additionally increasing the oxygen partial pressure was able to reducethe number of electron carriers.

For example, as shown in FIG. 3, an InGaO₃(ZnO)₄ thin film formed at asubstrate temperature of 25° C. and an oxygen partial pressure of 10⁻¹Pa had an electric conductivity additionally reduced to about 10⁻¹⁰S/cm. In addition, an InGaO₃(ZnO)₄ thin film formed at an oxygen partialpressure in excess of 10⁻¹ Pa had so high an electrical resistance thatits electric conductivity could not be measured. In this case, theelectron mobility, which could not be measured, was estimated to beabout 1 cm²/V·sec as a result of extrapolation from a value in a filmhaving a large electron carrier concentration.

That is, it was able to constitute a normally-off transistor having anon-off ratio in excess of 10³ by using a transparent amorphous oxidethin film constituted by In—Ga—Zn—O produced by means of a sputteringdeposition method in an argon gas atmosphere having an oxygen partialpressure in excess of 3×10⁻² Pa, or desirably in excess of 5×10⁻¹ Pa andhaving a composition in a crystalline state represented byInGaO₃(ZnO)_(m) (where m represents a natural number of less than 6).

When the apparatus and the material shown in this embodiment are used,the oxygen partial pressure upon film formation by means of sputteringis, for example, in the range of 3×10⁻² Pa to 5×10⁻¹ Pa (bothinclusive). As shown in FIG. 2, the electron mobility increases withincreasing number of conduction electrons in a thin film produced bymeans of each of the pulse laser deposition method and the sputteringmethod.

As described above, controlling an oxygen partial pressure can reducethe number of oxygen defects, thereby reducing an electron carrierconcentration. In addition, in an amorphous state, unlike apolycrystalline state, substantially no particle interface is present,so an amorphous thin film having a high electron mobility can beobtained.

It should be noted that an InGaO₃(ZnO)₄ amorphous oxide film obtained byusing a polyethylene terephthalate (PET) film having a thickness of 200μm instead of a glass substrate also showed similar characteristics.

The use of a polycrystal InGaO₃(Zn_(1-x)Mg_(x)O)_(m) (where m representsa natural number of less than 6 and 0<x≦1) as a target can provide ahigh-resistance amorphous InGaO₃(Zn_(1-x)Mg_(x)O)_(m) film even at anoxygen partial pressure of less than 1 Pa.

For example, when a target obtained by substituting Zn by 80 at. % of Mgis used, the electron carrier concentration of a film obtained by meansof a pulse laser deposition method in an atmosphere having an oxygenpartial pressure of 0.8 Pa can be less than 10¹⁶/cm³ (the electricalresistance is about 10⁻² S/cm)

The electron mobility of such film reduces as compared to a film with noadditional Mg, but the degree of the reduction is small: the electronmobility at room temperature is about 5 cm²/(V·sec), which is about oneorder of magnitude larger than that of amorphous silicon. Upon filmformation under the same conditions, the electric conductivity and theelectron mobility reduce with increasing Mg content. Therefore, the Mgcontent is preferably in excess of 20% and less than 85% (that is,0.2<x<0.85).

In the thin film transistor using the above-described amorphous oxidefilm, one of Al₂O₃, Y₂O₃, and HfO₂, or a mixed crystal compoundcontaining at least two kinds of these compounds is preferably used fora gate insulation film.

When a defect is present in an interface between the gate insulationthin film and the channel layer thin film, electron mobility reduces andhysteresis occurs in transistor characteristics. In addition, a leakcurrent varies to a large extent depending on the kind of the gateinsulation film. Therefore, a gate insulation film suitable for achannel layer needs to be selected. The use of an Al₂O₃ film can reducea leak current. In addition, the use of a Y₂O₃ film can reducehysteresis. Furthermore, the use of an HfO₂ film having a highdielectric constant can increase electron mobility. In addition, the useof those films can result in the formation of a TFT having a small leakcurrent, small hysteresis, and a large electron mobility. In addition,each of a gate insulation film forming process and a channel layerforming process can be performed at room temperature, so each of astaggered structure and an inversely staggered structure can be formedas a TFT structure.

The TFT thus formed is a three-terminal device equipped with a gateterminal, a source terminal, and a drain terminal, and is an activedevice which uses a semiconductor thin film formed on an insulatingsubstrate such as a ceramic, glass, or plastic as a channel layer inwhich an electron or a hole moves, and provides a switching function fora current between the source terminal and the drain terminal by applyinga voltage to the gate terminal to control a current flowing in thechannel layer.

The fact that a desired electron carrier concentration can be achievedby controlling an oxygen defective amount is important in the presentinvention.

In the foregoing description, the amount of oxygen in an amorphous oxidefilm (oxygen defective amount) is controlled in an atmosphere containinga predetermined concentration of oxygen upon film formation. It is alsopreferable to control (reduce or increase) the oxygen defective amountby subjecting the oxide film to a post treatment in an atmospherecontaining oxygen after the film formation.

To effectively control the oxygen defective amount, the temperature inthe atmosphere containing oxygen is in the range of desirably 0° C. to300° C. (both inclusive), preferably 25° C. to 250° C. (both inclusive),or more preferably 100° C. to 200° C. (both inclusive).

Of course, the oxygen defective amount may be controlled in theatmosphere containing oxygen upon film formation and then controlledthrough a post treatment in the atmosphere containing oxygen after thefilm formation. In addition, the oxygen partial pressure may becontrolled not upon film formation but after the film formation througha post treatment in the atmosphere containing oxygen as long as adesired electron carrier concentration (less than 10¹⁸/cm³) can beobtained.

The lower limit for the electron carrier concentration in the presentinvention, which varies depending on what kind of device, circuit, orapparatus an oxide film to be obtained is used for, is, for example,10¹⁴/cm³ or higher.

(Expansion of Material System)

As a result of research on an expanded composition system, it has beenfound that an amorphous oxide film having a small electron carrierconcentration and a large electron mobility can be produced by means ofan amorphous oxide composed of an oxide of at least one element of Zn,In, and Sn.

It has also been found that the amorphous oxide film has a specificproperty with which the electron mobility increases with increasingnumber of conduction electrons.

A normally-off TFT excellent in transistor characteristics includingon-off ratio, saturation current in a pinch-off state, and switchingspeed can be produced by means of the film.

An oxide having any one of the following characteristics (a) to (h) canbe used in the present invention.

(a) An amorphous oxide having an electron carrier concentration of lessthan 10¹⁸/cm³ at room temperature.

(b) An amorphous oxide whose electron mobility increases with increasingelectron carrier concentration.

The term “room temperature” as used herein refers to a temperature ofabout 0° C. to 40° C. The term “amorphous” as used herein refers to acompound having only a halo pattern to be observed, and showing nospecific diffraction ray, in an X-ray diffraction spectrum. The term“electron mobility” as used herein refers to an electron mobilitymeasured through Hall effect measurement.

(c) An amorphous oxide according to the above item (a) or (b) having anelectron mobility in excess of 0.1 cm²/V·sec at room temperature.

(d) An amorphous oxide according to the above item (b) or (c) exhibitingdegenerate conduction. The term “degenerate conduction” as used hereinrefers to a state where thermal activation energy in the temperaturedependence of an electrical resistance is 30 meV or less.

(e) An amorphous oxide according any one of the above items (a) to (d)containing at least one element of Zn, In, and Sn as a constituent.

(f) An amorphous oxide film obtained by incorporating, into theamorphous oxide according to the above item (e), at least one element ofa Group II element M2 having an atomic number smaller than that of Zn(M2 represents Mg or Ca); a Group III element M3 having an atomic numbersmaller than that of In (M3 represents B, Al, Ga, or Y); a Group IVelement M4 having an atomic number smaller than that of Sn (M4represents Si, Ge, or Zr); a Group V element M5 (M5 represents V, Nb, orTa); and Lu and W.

(g) An amorphous oxide film according to any one of the above items (a)to (f), which is a single compound having a composition in a crystallinestate represented by In_(1-x)M3_(x)O₃(Zn_(1-y)M2_(y)O)_(m) (where 0≦x,y≦1 and m represents 0 or a natural number of less than 6) or a mixtureof compounds having different m's. M3 represents Ga or the like, and M2represents Mg or the like.

(h) An amorphous oxide film according to any one of the above items (a)to (g) which is arranged on a glass substrate, a metal substrate, aplastic substrate, or a plastic film.

The present invention relates to a field effect transistor using theamorphous oxide or amorphous oxide film described above for a channellayer.

An amorphous oxide film having an electron carrier concentration inexcess of 10¹⁵/cm³ and less than 10¹⁸/cm³ is used for a channel layer toconstitute a field effect transistor in which a source terminal, a drainterminal, and a gate terminal are arranged via a gate insulation film.When a voltage of about 5 V is applied between the source and drainterminals, a current between the source and drain terminals with no gatevoltage applied can be about 10⁻⁷ A.

The electron mobility of an oxide crystal increases as the degree towhich the s orbitals of metal ions overlap with each other increases.The oxide crystal of Zn, In, or Sn having a large atomic number has alarge electron mobility of 0.1 to 200 cm²/(V·sec).

Furthermore, in the oxide, oxygen and a metal ion bond to each otherthrough an ionic bond.

As a result, even in an amorphous state in which a chemical bond has nodirectivity, a structure is random, and the direction of bonding isnonuniform, the electron mobility can be comparable to the electronmobility in a crystalline state.

On the other hand, replacing Zn, In, or Sn with an element having asmall atomic number reduces the electron mobility. As a result, theelectron mobility of the amorphous oxide according to the presentinvention is about 0.01 cm²/(V·sec) to 20 cm²/(V·sec).

When a channel layer of a transistor is produced by means of theabove-described oxide, one of Al₂O₃, Y₂O₃, and HfO₂, or a mixed crystalcompound containing at least two kinds of these compounds is preferablyused for a gate insulation film.

When a defect is present in an interface between the gate insulationthin film and the channel layer thin film, electron mobility reduces andhysteresis occurs in transistor characteristics. In addition, leakcurrent varies to a large extent depending on the kind of the gateinsulation film. Therefore, a gate insulation film suitable for achannel layer needs to be selected. The use of an Al₂O₃ film can reduceleak current. In addition, the use of a Y₂O₃ film can reduce hysteresis.Furthermore, the use of an HfO₂ film having a high dielectric constantcan increase field effect mobility. In addition, the use of a filmcomposed of a mixed crystal of those compounds can result in theformation of a TFT having a small leak current, small hysteresis, and alarge field effect mobility. In addition, each of a gate insulation filmforming process and a channel layer forming process can be performed atroom temperature, so each of a staggered structure and an inverselystaggered structure can be formed as a TFT structure.

An In₂O₃ oxide film can be formed by means of a vapor phase method, andan amorphous film can be obtained by adding about 0.1 Pa of water to anatmosphere during film formation.

Although an amorphous film is hardly obtained from each of ZnO and SnO₂,an amorphous film can be obtained by adding about 20 at. % of In₂O₃ toZnO or by adding about 90 at. % of In₂O₃ to SnO₂. In particular, about0.1 Pa of a nitrogen gas is desirably introduced into the atmosphere inorder to obtain an Sn—In—O-based amorphous film.

The above amorphous film can have an additional element constituting acomposite oxide of at least one element of: a Group II element M2 havingan atomic number smaller than that of Zn (M2 represents Mg or Ca); aGroup III element M3 having an atomic number smaller than that of In (M3represents B, Al, Ga, or Y); a Group IV element M4 having an atomicnumber smaller than that of Sn (M4 represents Si, Ge, or Zr); a Group Velement M5 (M5 represents V, Nb, or Ta); and Lu and W.

The additional element can additionally stabilize the amorphous film atroom temperature. In addition, the addition can expand the compositionrange in which the amorphous film can be obtained.

In particular, the addition of B, Si, or Ge having strong covalency iseffective in stabilizing an amorphous phase, and a composite oxidecomposed of ions different from each other in ionic radius to a largeextent has a stabilized amorphous phase.

For example, a stable amorphous film is hardly obtained at roomtemperature unless In accounts for more than about 20 at. % of anIn—Zn—O system. However, the addition of Mg in an amount equivalent tothat of In can provide a stable amorphous film when In accounts for morethan about 15 at. %.

An amorphous oxide film having an electron carrier concentration inexcess of 10¹⁵/cm³ and less than 10¹⁶/cm³ can be obtained by controllingan atmosphere in film formation by means of a vapor phase method.

An amorphous oxide is desirably formed into a film by means of any oneof the vapor phase methods such as a pulse laser deposition method (PLDmethod), a sputtering method (SP method), and an electron beamdeposition method. Of those vapor phase methods, a PLD method issuitable because the composition of a material system can be easilycontrolled, and an SP method is suitable in terms of mass productivity.However, a film forming method is not limited to those methods.

(Formation of In—Zn—Ga—O-Based Amorphous Oxide Film by Means of PLDMethod)

Polycrystalline sintered materials each having an InGaO₃(ZnO)composition or an InGaO₃(ZnO)₄ composition were used as targets todeposit an In—Zn—Ga—O-based amorphous oxide film on a glass substrate(1737 manufactured by Corning Inc.) by means of a PLD method using a KrFexcimer laser.

The film forming apparatus used was that shown in FIG. 9 describe above,and film forming conditions were the same as those in the case where theapparatus was used.

The substrate temperature was 25° C. X-ray diffraction was conducted oneach of the resultant films by means of a small angle X-ray scatteringmethod (SAXS) (thin film method, angle of incidence 0.5 degree). As aresult, no clear diffraction peak was detected. Therefore, each of theIn—Zn—Ga—O-based films produced from two kinds of targets was found tobe an amorphous film.

Furthermore, X-ray reflectance measurement was performed on each of theIn—Zn—Ga—O-based amorphous oxide films on the glass substrate, andpattern analysis was performed. As a result, each of the thin films wasfound to have a mean square roughness (Rrms) of about 0.5 nm and athickness of about 120 nm.

X-ray fluorescence (XRF) analysis confirmed that the metal compositionratio of the film obtained by using the polycrystalline sinteredmaterial having the InGaO₃(ZnO) composition as a target wasIn:Ga:Zn=1.1:1.1:0.9 and the metal composition ratio of the filmobtained by using the polycrystalline sintered material having theInGaO₃(ZnO)₄ composition as a target was In:Ga:Zn=0.98:1.02:4.

The electron carrier concentration of the amorphous oxide film obtainedby using the polycrystalline sintered material having the InGaO₃(ZnO)₄composition as a target was measured with the oxygen partial pressure ofthe atmosphere during film formation changed. FIG. 1 shows the results.Film formation in an atmosphere having an oxygen partial pressure inexcess of 4.2 Pa was able to reduce the electron carrier concentrationto less than 10¹⁸/cm³. In this case, the substrate had a temperaturemaintained at a temperature nearly equal to room temperature unlessintentionally heated. When the oxygen partial pressure was less than 6.5Pa, the surface of the resultant amorphous oxide film was flat.

When the oxygen partial pressure was 5 Pa, the amorphous oxide filmobtained by using the polycrystalline sintered material having theInGaO₃(ZnO)₄ composition as a target had an electron carrierconcentration of 10¹⁶/cm³ and an electric conductivity of 10⁻² S/cm. Inaddition, its electron mobility was estimated to be about 5 cm²/V sec.Owing to the analysis of a light absorption spectrum, the forbidden bandenergy width of the produced amorphous oxide film was determined to beabout 3 eV.

Additionally increasing the oxygen partial pressure was able toadditionally reduce the electron carrier concentration. As shown in FIG.1, an In—Zn—Ga—O-based amorphous oxide film formed at a substratetemperature of 25° C. and an oxygen partial pressure of 6 Pa had anelectron carrier concentration reduced to 8×10¹⁵/cm³ (electricconductivity: about 8×10⁻³ S/cm). The electron mobility of the resultantfilm was estimated to be in excess of 1 cm²/(V·sec). However, in the PLDmethod, when the oxygen partial pressure was 6.5 Pa or more, the surfaceof the deposited film became irregular, so it became difficult to usethe film as a channel layer of a TFT.

Investigation was made into the relationship between the electroncarrier concentration and electron mobility of each of In—Zn—Ga—O-basedamorphous oxide films formed at different oxygen partial pressures byusing the polycrystalline sintered material having the InGaO₃(ZnO)₄composition as a target. FIG. 2 shows the results. It was found that theelectron mobility increased from about 3 cm²/(V·sec) to about 11cm²/(V·sec) as the electron carrier concentration increased from10¹⁶/cm³ to 10²⁰/cm³. A similar tendency was observed in an amorphousoxide film obtained by using the polycrystalline sintered materialhaving the InGaO₃(ZnO) composition as a target.

An In—Zn—Ga—O-based amorphous oxide film obtained by using apolyethylene terephthalate (PET) film having a thickness of 200 μminstead of a glass substrate also showed similar characteristics.

(Formation of In—Zn—Ga—Mg—O-Based Amorphous Oxide Film by Means of PLDMethod)

A polycrystal InGaO₃(Zn_(1-x)Mg_(x)O)₄ (0<x≦1) was used as a target toform an InGaO₃(Zn_(1-x)Mg_(x)O)₄ (0<x≦1) film on a glass substrate bymeans of the PLD method.

The apparatus shown in FIG. 9 was used as a film forming apparatus.

An SiO₂ glass substrate (1737 manufactured by Corning Inc.) was preparedas a deposition substrate. The substrate was subjected to degreasingwashing by means of an ultrasonic wave for 5 minutes in each of acetone,ethanol, and ultrapure water as a pretreatment, and was then dried inthe air at 100° C. An InGa(Zn_(1-x)Mg_(x)O)₄ (x=1 to 0) sinteredmaterial (having a diameter of 20 mm and a thickness of 5 mm in size)was used as a target.

The target was produced by wet-mixing 4N reagents of In₂O₃, Ga₂O₃, ZnO,and MgO as starting materials in ethanol as a solvent; calcining themixture at 1,000° C. for 2 hours; dry-pulverizing the resultant; andsintering the pulverized product at 1,550° C. for 2 hours.

The ultimate pressure in the growth chamber was 2×10⁻⁶ (Pa), and theoxygen partial pressure during growth was set to be 0.8 (Pa). Thesubstrate temperature was room temperature (25° C.), and the distancebetween the target and the deposition substrate was 30 (mm).

The KrF excimer laser had a power of 1.5 (mJ/cm²/pulse), a pulse widthof 20 (nsec), a pulse rate of 10 (Hz), and an irradiation spot diameterof 1×1 (mm square).

The film forming rate was 7 (nm/min).

The oxygen partial pressure of the atmosphere was 0.8 Pa, and thesubstrate temperature was 25° C. X-ray diffraction was conducted on theresultant film by means of a small angle X-ray scattering method (SAXS)(thin film method, angle of incidence 0.5 degree). As a result, no cleardiffraction peak was detected. Therefore, the producedIn—Zn—Ga—Mg—O-based film was found to be an amorphous film. The surfaceof the resultant film was flat.

Targets having different values of x were used to determine the x valuedependence of each of the electric conductivity, electron carrierconcentration, and electron mobility of the In—Zn—Ga—Mg—O-basedamorphous oxide film formed in an atmosphere having an oxygen partialpressure of 0.8 Pa.

FIG. 4 shows the results. When the value of x exceeded 0.4, the electroncarrier concentration of an amorphous oxide film formed by means of thePLD method in an atmosphere having an oxygen partial pressure of 0.8 Pawas less than 10¹⁸/cm³. In addition, an amorphous oxide film having avalue of x in excess of 0.4 had an electron mobility in excess of 1cm²/V·sec.

As shown in FIG. 4, when a target obtained by substituting Zn by 80 at.% of Mg is used, the electron carrier concentration of a film obtainedby means of a pulse laser deposition method in an atmosphere having anoxygen partial pressure of 0.8 Pa can be less than 10¹⁶/cm³ (theelectrical resistance is about 10⁻² S/cm). The electron mobility of suchfilm reduces as compared to a film with no additional Mg, but the degreeof the reduction is small: the electron mobility at room temperature isabout 5 cm²/(V·sec), which is about one order of magnitude larger thanthat of amorphous silicon. Upon film formation under the sameconditions, the electric conductivity and the electron mobility reducewith increasing Mg content. Therefore, the Mg content is preferably inexcess of 20 at. % and less than 85 at. % (that is, 0.2<x<0.85), morepreferably 0.5<x<0.85.

An InGaO₃(Zn_(1-x)Mg_(x)O)₄ (0<x≦1) amorphous oxide film obtained byusing a polyethylene terephthalate (PET) film having a thickness of 200μm instead of a glass substrate also showed similar characteristics.

(Formation of In₂O₃ Amorphous Oxide Film by Means of PLD Method)

An In₂O₃ polycrystalline sintered material was used as a target to forman In₂O₃ film on a PET film having a thickness of 200 μm by means of thePLD method using a KrF excimer laser. The apparatus shown in FIG. 9 wasused. An SiO₂ glass substrate (1737 manufactured by Corning Inc.) wasprepared as a deposition substrate.

The substrate was subjected to degreasing washing by means of anultrasonic wave for 5 minutes in each of acetone, ethanol, and ultrapurewater as a pretreatment, and was then dried in the air at 100° C.

An In₂O₃ sintered material (having a diameter of 20 mm and a thicknessof 5 mm in size) was used as a target. The target was prepared bycalcining a 4N reagent of In₂O₃ as a starting material at 1,000° C. for2 hours; dry-pulverizing the resultant; and sintering the pulverizedproduct at 1,550° C. for 2 hours.

The ultimate pressure in the growth chamber was 2×10U−6 (Pa), and theoxygen partial pressure during growth and the substrate temperature wereset to be 5 (Pa) and room temperature, respectively.

The oxygen partial pressure and the vapor partial pressure were set tobe 5 Pa and 0.1 Pa, respectively, and 200 W was applied to anoxygen-radical-generating apparatus to generate an oxygen radical.

The distance between the target and the deposition substrate was 40(mm). The KrF excimer laser had a power of 0.5 (mJ/cm²/pulse), a pulsewidth of 20 (nsec), a pulse rate of 10 (Hz), and an irradiation spotdiameter of 1×1 (mm square).

The film forming rate was 3 (nm/min).

X-ray diffraction was conducted on the resultant film by means of asmall angle X-ray scattering method (SAXS) (thin film method, angle ofincidence 0.5 degree). As a result, no clear diffraction peak wasdetected. Therefore, the produced In—O-based film was found to be anamorphous film. The film had a thickness of 80 nm.

The resultant In—O-base amorphous oxide film had an electron carrierconcentration of 5×10¹⁷/cm³ and an electron mobility of about 7cm²/V·sec.

(Formation of In—Sn—O-Based Amorphous Oxide Film by Means of PLD Method)

An (In_(0.9)Sn_(0.1))O_(3.1) polycrystalline sintered material was usedas a target to form an In—Sn—O-based oxide film on a PET film having athickness of 200 μm by means of the PLD method using a KrF excimerlaser.

To be specific, an SiO₂ glass substrate (1737 manufactured by CorningInc.) was prepared as a deposition substrate.

The substrate was subjected to degreasing washing by means of anultrasonic wave for 5 minutes in each of acetone, ethanol, and ultrapurewater as a pretreatment, and was then dried in the air at 100° C.

An In₂O₃—SnO₂ sintered material (having a diameter of 20 mm and athickness of 5 mm in size) was prepared as a target. The target wasproduced by wet-mixing a 4N reagent of In₂O₃—SnO₂ as a starting materialin ethanol as a solvent; calcining the mixture at 1,000° C. for 2 hours;dry-pulverizing the resultant; and sintering the pulverized product at1,550° C. for 2 hours.

The substrate temperature was room temperature. The oxygen partialpressure and the nitrogen partial pressure were set to be 5 (Pa) and 0.1(Pa), respectively, and 200 W was applied to anoxygen-radical-generating apparatus to generate an oxygen radical.

The distance between the target and the deposition substrate was 30(mm). The KrF excimer laser had a power of 1.5 (mJ/cm²/pulse), a pulsewidth of 20 (nsec), a pulse rate of 10 (Hz), and an irradiation spotdiameter of 1×1 (mm square).

The film forming rate was 6 (nm/min).

X-ray diffraction was conducted on the resultant film by means of asmall angle X-ray scattering method (SAXS) (thin film method, angle ofincidence 0.5 degree). As a result, no clear diffraction peak wasdetected. Therefore, the produced In—Sn—O-based film was found to be anamorphous film.

The resultant In—Sn—O amorphous oxide film had an electron carrierconcentration of 8×10¹⁷/cm³, an electron mobility of about 5 cm²/V·sec,and a thickness of 100 nm.

(Formation of In—Ga—O-Based Amorphous Oxide Film by Means of PLD Method)

An SiO₂ glass substrate (1737 manufactured by Corning Inc.) was preparedas a deposition substrate.

The substrate was subjected to degreasing washing by means of anultrasonic wave for 5 minutes in each of acetone, ethanol, and ultrapurewater as a pretreatment, and was then dried in the air at 100° C.

An (In₂O₃)_(1-x)—(Ga₂O₃)_(x) (x=0 to 1) sintered material (having adiameter of 20 mm and a thickness of 5 mm in size) was prepared as atarget. For example, in the case of x=0.1, the target is(In_(0.9)Ga_(0.1))₂O₃ polycrystalline sintered material.

The target was produced by wet-mixing a 4N reagent of In₂O₃—Ga₂O₂ as astarting material in ethanol as a solvent; calcining the mixture at1,000° C. for 2 hours; dry-pulverizing the resultant; and sintering thepulverized product at 1,550° C. for 2 hours.

The ultimate pressure in the growth chamber was 2×10⁻⁶ (Pa), and theoxygen partial pressure during growth was set to be 1 (Pa).

The substrate temperature was room temperature. The distance between thetarget and the deposition substrate was 30 (mm). The KrF excimer laserhad a power of 1.5 (mJ/cm²/pulse), a pulse width of 20 (nsec), a pulserate of 10 (Hz), and an irradiation spot diameter of 1×1 (mm square).The film forming rate was 6 (nm/min).

The substrate temperature was 25° C. The oxygen partial pressure was 1Pa. X-ray diffraction was conducted on the resultant film by means of asmall angle X-ray scattering method (SAXS) (thin film method, angle ofincidence 0.5 degree). As a result, no clear diffraction peak wasdetected. Therefore, the produced In—Ga—O-based film was found to be anamorphous film. The film had a thickness of 120 nm.

The resultant In—Ga—O amorphous oxide film had an electron carrierconcentration of 8×10¹⁶/cm³ and an electron mobility of about 1 cm²/Vsec.

(Production of TFT Device Using In—Zn—Ga—O-Based Amorphous Oxide Film(Glass Substrate))

Production of TFT Device

A top gate TFT device shown in FIG. 5 was produced.

At first, a polycrystalline sintered material having an InGaO₃(ZnO)₄composition was used as a target to form an In—Ga—Zn—O-based amorphousoxide film on a glass substrate (1) at an oxygen partial pressure of 5Pa by means of the above-described PLD apparatus. Thus, anIn—Ga—Zn—O-based amorphous film having a thickness of 120 nm to be usedas a channel layer (2) was formed.

An In—Ga—Zn—O-based amorphous film and a gold film each having a largeelectric conductivity and a thickness of 30 nm were laminated on thefilm by means of the PLD method with the oxygen partial pressure in thechamber set to be less than 1 Pa, to thereby form a drain terminal (5)and a source terminal (6) by means of a photolithography method and alift-off method.

Finally, a Y₂O₃ film to be used as a gate insulation film (3)(thickness: 90 nm, relative dielectric constant: about 15, leak currentdensity: 10⁻³ A/cm² upon application of 0.5 MV/cm) was formed by meansof an electron beam deposition method. A gold film was formed on theY₂O₃ film, to thereby form a gate terminal (4) by means of aphotolithography method and a lift-off method. The channel length was 50μm and the channel width was 200 μm.

Evaluation of TFT Device for Characteristics

FIG. 6 shows the current-voltage characteristics of a TFT devicemeasured at room temperature. The fact that a drain current I_(DS)increased with increasing drain voltage V_(DS) shows that the conductionof the channel is of an n-type.

This is not in contradiction to the fact that an amorphousIn—Ga—Zn—O-based amorphous oxide film is an n-type conductor. I_(DS)saturated (pinched off) at V_(DS) of about 6 V. The saturation is atypical behavior of a semiconductor transistor. Investigation into again characteristic showed that the threshold value for a gate voltageV_(GS) was about −0.5 V upon application of V_(DS)=4 V.

A current I_(DS)=1.0×10⁻⁵ A flowed when V_(G)=10 V. This corresponds tothe fact that a gate bias enabled a carrier to be induced in anIn—Ga—Zn—O-based amorphous oxide film as an insulator.

The transistor had an on-off ratio in excess of 10³. The field effectmobility was calculated from an output characteristic. As a result, afield effect mobility of about 7 cm²(Vs)⁻¹ was obtained in a saturationregion. The produced device was irradiated with visible light to performsimilar measurement. However, no changes in transistor characteristicswere observed.

An amorphous oxide having an electron carrier concentration of less than10¹⁸/cm³ is applicable to a channel layer of a TFT. The electron carrierconcentration was more preferably 10¹⁷/cm³ or less, or still morepreferably 10¹⁶/cm³ or less.

(Production of TFT Device Using In—Zn—Ga—O-Based Amorphous Oxide Film(Amorphous Substrate))

A top gate TFT device shown in FIG. 5 was produced. At first, apolycrystalline sintered material having an InGaO₃(ZnO) composition wasused as a target to form an In—Zn—Ga—O-based amorphous oxide film havinga thickness of 120 nm to be used as a channel layer (2) on apolyethylene terephthalate (PET) film (1) at an oxygen partial pressureof 5 Pa by means of the PLD method.

An In—Zn—Ga—O-based amorphous oxide film and a gold film each having alarge electric conductivity and a thickness of 30 nm were laminated onthe film by means of the PLD method with the oxygen partial pressure inthe chamber set to be less than 1 Pa, to thereby form a drain terminal(5) and a source terminal (6) by means of a photolithography method anda lift-off method. Finally, a gate insulation film (3) was formed bymeans of an electron beam deposition method, and a gold film was formedon the film to thereby form a gate terminal (4) by means of aphotolithography method and a lift-off method. The channel length was 50μm and the channel width was 200 μm. Each of Y₂O₃ (thickness: 140 nm),Al₂O₃ (thickness: 130 μm), and HfO₂ (thickness: 140 μm) was used as agate insulation film to produce three kinds of TFT's each having theabove structure.

Evaluation of TFT Device for Characteristics

The current-voltage characteristics of a TFT formed on the PET filmmeasured at room temperature were the same as those shown in FIG. 6.That is, the fact that a drain current I_(DS) increased with increasingdrain voltage V_(DS) shows that the conduction of the channel is of ann-type. This is not in contradiction to the fact that an amorphousIn—Ga—Zn—O-based amorphous oxide film is an n-type conductor. I_(DS)saturated (pinched off) at V_(DS) of about 6 V. The saturation is atypical behavior of a transistor. A current I_(ds)=10⁻⁸ A flowed whenV_(g)=0, while a current I_(DS)=2.0×10⁻⁵ A flowed when V_(g)=10 V. Thiscorresponds to the fact that a gate bias enabled an electron carrier tobe induced in an In—Ga—Zn—O-based amorphous oxide film as an insulator.

The transistor had an on-off ratio in excess of 10³. The field effectmobility was calculated from an output characteristic. As a result, afield effect mobility of about 7 cm² (Vs)⁻¹ was obtained in a saturationregion.

The device produced on the PET film was bent at a radius of curvature of30 mm to perform similar measurement of transistor characteristics.However, no changes in transistor characteristics were observed. Thedevice was irradiated with visible light to perform similar measurement.However, no changes in transistor characteristics were observed.

The TFT using an Al₂O₃ film as a gate insulation film showed transistorcharacteristics similar to those shown in FIG. 6. A current I_(ds)=10⁻⁸A flowed when V_(g)=0, while a current I_(DS)=5.0×10⁻⁶ A flowed whenV_(g)=10 V. The transistor had an on-off ratio in excess of 10². Thefield effect mobility was calculated from an output characteristic. As aresult, a field effect mobility of about 2 cm²(Vs)⁻¹ was obtained in asaturation region.

The TFT using an HfO₂ film as a gate insulation film showed transistorcharacteristics similar to those shown in FIG. 6. A current I_(ds)=10⁻⁸A flowed when V_(g)=0, while a current I_(DS)=1.0×10⁻⁶ A flowed whenV_(g)=10 V. The transistor had an on-off ratio in excess of 10². A fieldeffect mobility was calculated from an output characteristic. As aresult, a field effect mobility of about 10 cm²(Vs)⁻¹ was obtained in asaturation region.

(Production of TFT Device Using In₂O₃ Amorphous Oxide Film by Means ofPLD Method)

A top gate TFT device shown in FIG. 5 was produced. At first, an In₂O₃amorphous oxide film having a thickness of 80 nm to be used as a channellayer (2) was formed on a polyethylene terephthalate (PET) film (1) bymeans of the PLD method.

Then, an In₂O₃ amorphous oxide film and a gold film each having a largeelectric conductivity and a thickness of 30 nm were laminated on thefilm by means of the PLD method with the oxygen partial pressure in thechamber set to be less than 1 Pa and a voltage to be applied to anoxygen-radical-generating apparatus set to zero, to thereby form a drainterminal (5) and a source terminal (6) by means of a photolithographymethod and a lift-off method. Finally, a Y₂O₃ film to be used as a gateinsulation film (3) was formed by means of an electron beam depositionmethod, and a gold film was formed on the film to thereby form a gateterminal (4) by means of a photolithography method and a lift-offmethod.

Evaluation of TFT Device for Characteristics

The current-voltage characteristics of the TFT formed on the PET filmwere measured at room temperature. The fact that a drain current I_(DS)increased with increasing drain voltage V_(DS) shows that the channel isan n-type semiconductor. This is not in contradiction to the fact thatan In—O-based amorphous oxide film is an n-type conductor. I_(DS)saturated (pinched off) at V_(DS) of about 5 V. The saturation is atypical behavior of a transistor. A current I_(DS)=2×10⁻⁸ A flowed whenV_(g)=0 V, while a current I_(DS)=2.0×10⁻⁶ A flowed when V_(G)=10 V.This corresponds to the fact that a gate bias enabled an electroncarrier to be induced in an In—O-based amorphous oxide film as aninsulator.

The transistor had an on-off ratio of about 10². The field effectmobility was calculated from an output characteristic. As a result, afield effect mobility of about 10 cm² (Vs)⁻¹ was obtained in asaturation region. A TFT device produced on a glass substrate showedsimilar characteristics.

The device produced on the PET film was bent at a radius of curvature of30 mm to perform similar measurement of transistor characteristics.However, no changes in transistor characteristics were observed.

(Production of TFT Device Using In—Sn—O-Based Amorphous Oxide Film byMeans of PLD Method)

A top gate TFT device shown in FIG. 5 was produced. At first, anIn—Sn—O-based amorphous oxide film having a thickness of 100 nm to beused as a channel layer (2) was formed on a polyethylene terephthalate(PET) film (1) by means of the PLD method. Then, an In—Sn—O-basedamorphous oxide film and a gold film each having a large electricconductivity and a thickness of 30 nm were laminated on the film bymeans of the PLD method with the oxygen partial pressure in the chamberset to be less than 1 Pa and a voltage to be applied to anoxygen-radical-generating apparatus set to zero, to thereby form a drainterminal (5) and a source terminal (6) by means of a photolithographymethod and a lift-off method. Finally, a Y₂O₃ film to be used as a gateinsulation film (3) was formed by means of an electron beam depositionmethod, and a gold film was formed on the film to thereby form a gateterminal (4) by means of a photolithography method and a lift-offmethod.

Evaluation of TFT Device for Characteristics

The current-voltage characteristics of the TFT formed on the PET filmwere measured at room temperature. The fact that a drain current I_(DS)increased with increasing drain voltage V_(DS) shows that the channel isan n-type semiconductor. This is not in contradiction to the fact thatan In—Sn—O-based amorphous oxide film is an n-type conductor. I_(DS)saturated (pinched off) at V_(DS) of about 6 V. The saturation is atypical behavior of a transistor. A current I_(DS)=5×10⁻⁸ A flowed whenV_(g)=0 V, while a current I_(DS)=5.0×10⁻⁵ A flowed when V_(G)=10 V.This corresponds to the fact that a gate bias enabled an electroncarrier to be induced in an In—Sn—O-based amorphous oxide film as aninsulator.

The transistor had an on-off ratio of about 10³. The field effectmobility was calculated from an output characteristic. As a result, afield effect mobility of about 5 cm² (Vs)⁻¹ was obtained in a saturationregion. A TFT device produced on a glass substrate showed similarcharacteristics.

The device produced on the PET film was bent at a radius of curvature of30 mm to perform similar measurement of transistor characteristics.However, no changes in transistor characteristics were observed.

(Production of TFT Device Using In—Ga—O-Based Amorphous Oxide Film byMeans of PLD Method)

A top gate TFT device shown in FIG. 5 was produced. At first, anIn—Ga—O-based amorphous oxide film having a thickness of 120 nm to beused as a channel layer (2) was formed on a polyethylene terephthalate(PET) film (1) by means of a film forming method shown in Example 6.Then, an In—Ga—O-based amorphous oxide film and a gold film each havinga large electric conductivity and a thickness of 30 nm were laminated onthe film by means of the PLD method with the oxygen partial pressure inthe chamber set to be less than 1 Pa and a voltage to be applied to anoxygen-radical-generating apparatus set to zero, to thereby form a drainterminal (5) and a source terminal (6) by means of a photolithographymethod and a lift-off method. Finally, a Y₂O₃ film to be used as a gateinsulation film (3) was formed by means of an electron beam depositionmethod, and a gold film was formed on the film to thereby form a gateterminal (4) by means of a photolithography method and a lift-offmethod.

Evaluation of TFT Device for Characteristics

The current-voltage characteristics of the TFT formed on the PET filmwere measured at room temperature. The fact that a drain current I_(DS)increased with increasing drain voltage V_(DS) shows that the channel isan n-type semiconductor. This is not in contradiction to the fact thatan In—Ga—O-based amorphous oxide film is an n-type conductor. I_(DS)saturated (pinched off) at V_(DS) of about 6 V. The saturation is atypical behavior of a transistor. A current I_(DS)=1×10⁻⁸ A flowed whenV_(g)=0 V, while a current I_(DS)=1.0×10⁻⁶ A flowed when V_(G)=10 V.This corresponds to the fact that a gate bias enabled an electroncarrier to be induced in an In—Ga—O-based amorphous oxide film as aninsulator.

The transistor had an on-off ratio of about 10². The field effectmobility was calculated from an output characteristic. As a result, afield effect mobility of about 0.8 cm² (Vs)⁻¹ was obtained in asaturation region. A TFT device produced on a glass substrate showedsimilar characteristics.

The device produced on the PET film was bent at a radius of curvature of30 mm to perform similar measurement of transistor characteristics.However, no changes in transistor characteristics were observed.

An amorphous oxide having an electron carrier concentration of less than10¹⁸/cm³ is applicable to a channel layer of a TFT. The electron carrierconcentration was more preferably 10¹⁷/cm³ or less, or still morepreferably 10¹⁶/cm³ or less.

Second Embodiment

The present invention also relates to a light control device obtained byconnecting, to a drain as an output terminal of a field effect TFT, aninput electrode of a light-emitting device such as an electroluminescentdevice; or a light transmittance control device or a light reflectancecontrol device composed of a liquid crystal cell or an electrophoreticparticle cell.

Description will be made with reference to FIG. 7.

A TFT is constituted by an amorphous oxide semiconductor film 12deposited and patterned on a substrate 11, a source electrode 13, adrain electrode 14, a gate insulation film 15, and a gate electrode 16.

An electrode 18 is connected to the drain electrode 14 via an interlayerinsulation film 17. The electrode 18 is in contact with a light-emittinglayer 19, and the light-emitting layer 19 is in contact with anelectrode 20.

A current to be injected into the light-emitting layer 19 is controlledon the basis of the value for a current flowing from the sourceelectrode 13 to the drain electrode 14 via a channel formed in theamorphous oxide semiconductor film 12. That is, the current can becontrolled by means of a voltage to be applied by the gate electrode 16.

Here, the light-emitting layer 19 is preferably an inorganic or organicelectroluminescent device.

In addition, as shown in FIG. 8, the drain electrode 14 may be extendedto serve as the electrode 18. That is, the drain electrode 14 may be theelectrode 18 for applying a voltage to a light transmittance controldevice or a light reflectance control device composed of a liquidcrystal cell or electrophoretic particle cell 23 sandwiched betweenhigh-resistance films 21 and 22.

With the above constitution, the voltage to be applied to the lighttransmittance control device or the light reflectance control device canbe controlled on the basis of the value for a current flowing from thesource electrode 13 to the drain electrode 14 via a channel formed inthe amorphous oxide semiconductor film 12.

That is, the voltage can be controlled by means of the voltage of thegate 6 of the TFT. The high-resistance films 21 and 22 are not neededwhen the light transmittance control device or the light reflectancecontrol device is a capsule obtained by sealing a fluid and a particleinto an insulating coat.

A representative structure for each of the TFT's in the above twoexamples is a top gate and coplanar structure. However, the presentinvention is not necessarily limited to the structure. Any otherstructure such as a staggered structure can also be used as long as adrain electrode as an output terminal of a TFT and a light-emittingdevice are connected so as to be topologically identical to each other.

Shown in each of the above two examples is an example in which a pair ofelectrodes for driving a light-emitting device, a light transmittancecontrol device, or a light reflectance control device is arranged inparallel with a substrate. However, the present invention is notnecessarily limited to the structure.

At least one of the electrodes may be arranged perpendicular to thesubstrate as long as the drain electrode as the output terminal of theTFT and the light-emitting device are connected so as to betopologically identical to each other.

Furthermore, only one TFT to be connected to a light-emitting device, alight transmittance control device, or a light reflectance controldevice is shown in each of the above two examples. However, the presentinvention is not necessarily limited to the structure.

The TFT shown in the figure may be connected to another TFT according tothe present invention as long the TFT shown in the figure corresponds tothe final stage of a circuit constituted by the TFT's.

When a pair of electrodes for driving a light-emitting device, a lighttransmittance control device, or a light reflectance control device isarranged in parallel with a substrate, an electrode of one of thelight-emitting device and the light reflectance control device needs tobe transparent with respect to a luminous wavelength or the wavelengthof reflected light. Alternatively, both electrodes of a lighttransmittance control device need to be transparent with respect totransmitted light. The term “transparent” as used herein refers to aconcept that includes of course one which is substantially transparentand one having light transmissivity.

Furthermore, all constituents of the TFT of the present invention may betransparent. In this case, a transparent light control device can beobtained. In addition, such light control device can be arranged on asubstrate having low heat resistance such as a light weight, flexible,and transparent plastic substrate made of a resin.

The multiple light control devices each of which is described in thefirst or second embodiment can be arranged two-dimensionally togetherwith the multiple TFT's wired in an active matrix manner.

For example, an active matrix circuit in which the gate electrode 5 ofone TFT for driving a light control device is connected to a gate wireof an active matrix and the source electrode of the TFT is wired to asignal destination is constituted. With the constitution, a displayusing each light control device as a pixel can be provided.

Furthermore, when multiple light control devices adjacent to each otherand different from each other in luminous wavelength, transmitted lightwavelength, or reflected light wavelength constitute one pixel, a colordisplay can be provided. In this case, of course, a color filter may beused.

Other Embodiments

The present invention also relates to a broadcasting dynamic imagedisplay device such as a television receiving set including theabove-described display. In particular, the display of the presentinvention provides a portable broadcasting dynamic image display with alight weight, flexibility, and safety with respect to breakage.

The present invention also relates to a digital information processingdevice such as a computer including the above-described display.

The display of the present invention has a light weight and is flexible,so it provides a stay-at-home computer display with the degree offreedom of arrangement and with portability. Furthermore, the displayprovides a portable digital information processing device such as anotebook computer or a personal digital assistant with a light weight,flexibility, and safety with respect to breakage.

The present invention also relates to a portable information equipmentsuch as a cellular phone, a portable music reproducer, a portabledynamic image reproducer, or a head mount display including theabove-described display. The display of the present invention providesany one of those portable information equipments with a light weight,flexibility, and safety with respect to breakage. In particular, whenthe display of the present invention which is made transparent is usedfor a head mount display, a see-through device can be provided.

The present invention also relates to an image pickup device such as astill camera or a movie camera including the above-described display.The display of the present invention provides any one of those imagepickup devices with a light weight, flexibility, and safety with respectto breakage.

The present invention also relates to a building structure such as awindow, a door, a ceiling, a floor, an inner wall, an outer wall, or apartition including the above-described display. Since the display ofthe present invention has a light weight and flexibility, and can bemade transparent, it can be easily attached to any one of those buildingstructures. In addition, the display does not impair the externalappearance of the building structure when no image is displayed.

The present invention also relates to a structure such as a window, adoor, a ceiling, a floor, an inner wall, an outer wall, or a partitionfor a movable body such as a vehicle, an airplane, or a ship includingthe above-described display.

Since the display of the present invention has a light weight andflexibility, and can be made transparent, it can be easily attached toany one of those building structures. In addition, the display does notimpair the external appearance of the building structure when no imageis displayed. When the display of the present invention which is madetransparent is used for a transparent window for monitoring andobserving the surroundings of a movable body, the display can display aninformation image if needed and does not inhibit the monitoring andobservation of the surroundings if such image is not needed.

The present invention also relates to an advertising device such asadvertising means in a vehicle of a public transportation, or asignboard or advertising tower in a city including the above-describeddisplay. The display of the present invention can not only alwaysreplace an invariable medium such as a printed article that has beenmainly used for any such adverting device heretofore but also display adynamic image.

EXAMPLES

Hereinafter, the examples of the present invention will be described.

At first, a method of forming a TFT using an amorphous oxide applicableto the present invention will be described.

(Formation of Amorphous In—Ga—Zn—O TFT)

A polycrystalline sintered material having an InGaO₃(ZnO)₄ compositionis used as a target to deposit an In—Ga—Zn—O-based amorphous oxidesemiconductor film having a thickness of 50 nm by means of a sputteringmethod on a polyether sulfine-based transparent plastic substrate withits surface treated.

The oxygen partial pressure in a chamber is 5×10⁻² Pa and the substratetemperature is 25° C. The amorphous oxide semiconductor film ispatterned into an island measuring 30 μm×15 μm by means of aphotolithography method.

Next, two kinds of islands measuring 35 μm×10 μm and 30 μm×10 μm eachcomposed of an ITO film having a thickness of 50 nm are formed by meansof the same film forming method and patterning method at the center ofthe island composed of the amorphous oxide semiconductor film at aninterval of 5 μm in parallel with the direction of the longer side ofthe island.

It should be noted that the ends in the directions of the longer sidesof those islands each composed of an ITO film are aligned with theisland composed of the amorphous oxide semiconductor film measuring 30μm×10 μm.

That is, the island composed of the amorphous oxide semiconductor filmmeasuring 30 μm×10 μm is in contact with each ITO film in a regionmeasuring 30 μm×5 μm at each end in the direction of the shorter side.

The island composed of an ITO film measuring 35 μm×10 μm extends off theisland composed of the amorphous oxide semiconductor film by 5 μm in thedirection of the shorter side and by 5 μm on one side in the directionof the longer side. The island composed of an ITO film measuring 30μm×10 μm extends off the island composed of the amorphous oxidesemiconductor film by 5 μm only in the direction of the shorter side.

Next, an island measuring 40 μm×15 μm composed of a Y₂O₃ film having athickness of 100 nm is similarly arranged on the island measuring 30μm×15 μm composed of the amorphous oxide semiconductor film with theircenters of gravity (and their longer sides) aligned with each other.

Finally, an island measuring 30 μm×5 μm composed of an ITO film having athickness of 50 nm is formed on the island measuring 30 μm×15 μmcomposed of the amorphous oxide semiconductor film with the longer sidesof the island composed of the ITO film in parallel with the center ofthe island composed of the amorphous oxide semiconductor film.

Through the above steps, the islands measuring 35 μm×10 μm and 30 μm×10μm each composed of an ITO film serve as a source electrode and a drainelectrode, respectively. The amorphous oxide semiconductor film placedat a gap between those islands serves as a channel region, and the Y₂O₃film serves as a gate insulation film. Then, the island measuring 30μm×5 μm composed of an ITO film at the uppermost portion serves as agate electrode. Thus, a field effect n-channel TFT is constituted.

Such TFT shows characteristics of a field effect mobility of 5cm²V⁻¹s⁻¹; a threshold voltage of 1 V; and an on-off ratio of about 10³or more.

Example 1 Production of Light Control Device Using the Above TFT

In the above TFT, a shorter side of the island composed of an ITO filmto serve as the drain electrode is extended up to 100 μm. The extended90-μm portion is left, and the TFT is coated with an insulating layerwith wiring to the source electrode and the gate electrode secured.

A polyimide film (orientation film) is applied to the layer to perform arubbing step.

Meanwhile, a plastic substrate having an ITO film and a polyimide filmformed thereon and subjected to a rubbing step is separately prepared.The above substrate on which the TFT has been formed and the separatelyprepared substrate are arranged so as to be opposite to each other witha gap of 5 μm between them. A nematic liquid crystal is injected intothe gap.

Furthermore, a pair of polarizing plates are arranged on both sides ofthe structure.

Here, when a voltage is applied to the source electrode of the TFT andthe voltage applied to the gate electrode is changed, the lighttransmittance of only a region measuring 30 μm×90 μm as part of theisland composed of an ITO film extended from the drain electrodechanges.

The transmittance can be continuously changed by the voltage between asource and a drain with a gate voltage with which the TFT is in onstate.

In this example, a white plastic substrate is used as a substrate onwhich a TFT is to be formed, and each electrode of the TFT is replacedwith gold. Then, none of a polyimide film and a polarizing plate isused, and a gap between white and transparent plastic substrates isfilled with a capsule obtained by coating a particle and a fluid with aninsulating coat. An electrophoretic particle is used as the particle.

The voltage between the extended drain electrode and the upper ITO filmis controlled by the above TFT. The vertical movement of the particle inthe capsule enables the reflectance of the extended drain electroderegion seen from the side of the transparent substrate to be controlled.

In this example, multiple TFT's are formed so as to be adjacent to eachother to constitute, for example, a current control circuit typicallyconstituted by four transistors and one capacitor. In addition, the TFTshown in FIG. 7 may be used for one of the transistors on the finalstage to drive a light-emitting device.

For example, the above-described TFT using an ITO film for a drainelectrode is used, and an organic electroluminescent device composed ofa charge-injecting layer and a light-emitting layer is formed in aregion measuring 30 μm×90 μm as part of the island composed of the ITOfilm extend from the drain electrode.

Example 2 Display Using the Above Light Control Device

The above light control devices are arranged two-dimensionally. Forexample, the light transmittance control device or light reflectancecontrol device of Example 1 is used.

7,425×1,790 light control devices each having an area of about 30 μm×115μm including its TFT are arranged in a square array at pitches of 40 μmand 120 μm in directions of shorter and longer sides, respectively.1,790 gate wires penetrating the gate electrodes of the 7,425 TFT's arearranged in the direction of the longer side. Then, 7,425 signal wirespenetrating the portions of the source electrodes of the 1,790 TFT'sextending off the island composed of the amorphous oxide semiconductorfilm by 5 μm are arranged.

The respective wires are connected to a gate driver circuit and a sourcedriver circuit. Furthermore, a color filter which is aligned at the samesize as that of each light control device and in which R, G, and Brepeat in the direction of the longer side is arranged on the surface.Thus, an A4-size active matrix color display at about 211 ppi can beconstituted.

In the light control device using the light-emitting device of Example 1as well, out of the four TFT's in one light control device, the gateelectrode of a first TFT is wired to a gate wire and the sourceelectrode of a second TFT is wired to a signal wire. Furthermore, theluminous wavelength of a light-emitting device is caused to repeat inthe direction of the longer side by R, G, and B, whereby alight-emitting color display having the same resolution can beconstituted.

Here, a driver circuit for driving an active matrix may be constitutedby using the TFT of the present invention which is the same as that of apixel, or an existing IC chip may be used for the circuit.

Example 3 Device Including the Above Display

The above display is provided with a device essential to a broadcastingdynamic image display device such as a broadcasting receiving device ora voice and image processing device, and the resultant is included in athin casing together with a power source and an interface. Thus, abroadcasting dynamic image display device having a light weight, a thinthickness, and high safety with respect to falling and an impact isprovided.

In addition, the above display is connected to a device essential to adigital information processing device such as a central processor, astorage device, or a network device, and the resultant is included in athin casing together with a power source and an interface. Thus, anintegrated digital information processing device having a light weight,a thin thickness, and high portability is provided.

In addition, the area and number of light control devices of the abovedisplay are reduced to about to 2 to 5 inches in a diagonal line. Thedisplay is connected to a device essential to a portable informationequipment such as a processor, a storage device, or a network device,and the resultant is included in a small and thin casing together with apower source and an interface. Thus, a portable information equipmenthaving a light weight, a small size, a thin thickness, and high safetywith respect to falling and an impact is provided.

In addition, a similar small display is connected to a device essentialto an image pickup device such as an imaging device, a storage device,or a signal processing device, and the resultant is included in a smalland light weight casing together with a power source and an interface.Thus, an image pickup device having a light weight, a small size, andhigh safety with respect to falling and an impact is provided.

In addition, oppositely, the display in which the size of one lightcontrol device is enlarged and the display area of which is enlarged isattached to or incorporated into any one of the above buildingstructures, whereby a building structure capable of displaying anarbitrary image is provided.

In addition, the display is incorporated as any one of the abovestructures for movable bodies, whereby a structure for a movable bodycapable of displaying an arbitrary image is provided.

In addition, the display is incorporated as part of any one of the aboveadvertising devices, whereby an advertising device capable of displayingan arbitrary image is provided.

The light control device and the display according to the presentinvention can find use in a wide variety of applications including abroadcasting dynamic image display device, a digital informationprocessing device, a portable information equipment, an image pickupdevice, a building structure, a structure for a movable body, and anadvertising device each of which has a light weight, a thin thickness,and high safety with respect to breakage.

This application claims priority from Japanese Patent Application No.2004-326682 filed on Nov. 10, 2004, which is hereby incorporated byreference herein.

1. An active matrix display comprising: a light control device; and afield effect transistor for driving the light control device, wherein anactive layer of the field effect transistor comprises an amorphous oxidehaving an electron carrier concentration of less than 10¹⁸/cm³.
 2. Anactive matrix display according to claim 1, wherein the amorphous oxidecomprises an oxide containing at least one of Zn, In, and Sn.
 3. Anactive matrix display according to claim 1, wherein the amorphous oxideis selected from the group consisting of an oxide containing In, Zn andSn, an oxide containing In and Zn, an oxide containing In and Sn, and anoxide containing In.
 4. An active matrix display according to claim 1,wherein the amorphous oxide comprises an oxide containing In, Ga, andZn.
 5. An active matrix display according to claim 1, wherein the lightcontrol device contains one of a liquid crystal and an electrophoreticparticle.
 6. An active matrix display according to claim 1, wherein thelight control device contains a liquid crystal, and has an orientationfilm and an insulation film arranged in this order from a side of theliquid crystal between the active layer and the liquid crystal.
 7. Anactive matrix display according to claim 1, wherein the light controldevice contains a liquid crystal, and has an orientation film and aninsulation film arranged in this order from a side of the liquid crystalbetween a gate electrode of the field effect transistor and the liquidcrystal.
 8. An active matrix display according to claim 5, wherein theinsulation film comprises one of a silicon oxide film and a siliconnitride film.
 9. An active matrix display according to claim 1, whereinthe light control device is arranged on a flexible resin substrate. 10.An active matrix display according to claim 1, wherein the light controldevice is arranged on a light transmissive substrate.
 11. An activematrix display comprising: a light control device; and a field effecttransistor for driving the light control device, wherein an electronmobility of an active layer of the field effect transistor tends toincrease with increasing electron carrier concentration.
 12. An activematrix display comprising: a light control device; and a field effecttransistor for driving the light control device, wherein an active layerof the field effect transistor has an amorphous oxide semiconductorcapable of realizing normally-off of the field effect transistor.
 13. Anactive matrix display according to claim 12, wherein the amorphous oxidesemiconductor has an electron carrier concentration of less than10¹⁸/cm³ sufficient to realize the normally-off.
 14. An active matrixdisplay according to claim 12, wherein the amorphous oxide semiconductoris selected from the group consisting of an oxide containing In, Zn andSn, an oxide containing In and Zn, an oxide containing In and Sn, and anoxide containing In.
 15. An active matrix display comprising: a firstelectrode; a second electrode; a liquid crystal interposed between thefirst and second electrodes; and a field effect transistor for drivingthe liquid crystal, wherein an active layer of the transistor comprisesan amorphous oxide, and wherein the transistor is a normally-offtransistor.
 16. An active matrix display according to claim 15, whereinthe amorphous oxide is selected from the group consisting of an oxidecontaining In, Zn and Sn, an oxide containing In and Zn, an oxidecontaining In and Sn, and an oxide containing In.
 17. An active matrixdisplay according to claim 16, further comprising an orientation filmand an insulation film arranged in this order from a side of the liquidcrystal between a gate electrode of the field effect transistor and theliquid crystal.